Add safe apps endpoint

[katspaugh]

Overview

Currently the web app is using the config service directly to fetch the Safe Apps list. This is the only dependency that we still have with the config service directly. Currently the client gateway already exposes the Safe Apps list endpoint so we can just rely also in the gateway to handle this

https://gnosis.github.io/safe-client-gateway/docs/routes/safe_apps/routes/index.html

Goals

• Add the safe apps list route to the gateway-sdk
• Use this version of the gateway-sdk to fetch the safe apps list
• Remove the dependency with config service in web app